The initial clone may be time-consuming for a large project, but you
will only need to clone once.

The clone command creates a new directory named after the project ("git"
or "linux-2.6" in the examples above). After you cd into this
directory, you will see that it contains a copy of the project files,
called the working tree, together with a special
top-level directory named ".git", which contains all the information
about the history of the project.

How to check out a different version of a project

Git is best thought of as a tool for storing the history of a collection
of files. It stores the history as a compressed collection of
interrelated snapshots of the project’s contents. In git each such
version is called a commit.

Those snapshots aren’t necessarily all arranged in a single line from
oldest to newest; instead, work may simultaneously proceed along
parallel lines of development, called branches, which may
merge and diverge.

A single git repository can track development on multiple branches. It
does this by keeping a list of heads which reference the
latest commit on each branch; the git-branch(1) command shows
you the list of branch heads:

$ git branch
* master

A freshly cloned repository contains a single branch head, by default
named "master", with the working directory initialized to the state of
the project referred to by that branch head.

Most projects also use tags. Tags, like heads, are
references into the project’s history, and can be listed using the
git-tag(1) command:

Tags are expected to always point at the same version of a project,
while heads are expected to advance as development progresses.

Create a new branch head pointing to one of these versions and check it
out using git-checkout(1):

$ git checkout -b new v2.6.13

The working directory then reflects the contents that the project had
when it was tagged v2.6.13, and git-branch(1) shows two
branches, with an asterisk marking the currently checked-out branch:

$ git branch
master
* new

If you decide that you’d rather see version 2.6.17, you can modify
the current branch to point at v2.6.17 instead, with

$ git reset --hard v2.6.17

Note that if the current branch head was your only reference to a
particular point in history, then resetting that branch may leave you
with no way to find the history it used to point to; so use this command
carefully.

Understanding History: Commits

Every change in the history of a project is represented by a commit.
The git-show(1) command shows the most recent commit on the
current branch:

As you can see, a commit shows who made the latest change, what they
did, and why.

Every commit has a 40-hexdigit id, sometimes called the "object name" or the
"SHA-1 id", shown on the first line of the "git show" output. You can usually
refer to a commit by a shorter name, such as a tag or a branch name, but this
longer name can also be useful. Most importantly, it is a globally unique
name for this commit: so if you tell somebody else the object name (for
example in email), then you are guaranteed that name will refer to the same
commit in their repository that it does in yours (assuming their repository
has that commit at all). Since the object name is computed as a hash over the
contents of the commit, you are guaranteed that the commit can never change
without its name also changing.

In fact, in Chapter 7, Git concepts we shall see that everything stored in git
history, including file data and directory contents, is stored in an object
with a name that is a hash of its contents.

Understanding history: commits, parents, and reachability

Every commit (except the very first commit in a project) also has a
parent commit which shows what happened before this commit.
Following the chain of parents will eventually take you back to the
beginning of the project.

However, the commits do not form a simple list; git allows lines of
development to diverge and then reconverge, and the point where two
lines of development reconverge is called a "merge". The commit
representing a merge can therefore have more than one parent, with
each parent representing the most recent commit on one of the lines
of development leading to that point.

The best way to see how this works is using the gitk(1)
command; running gitk now on a git repository and looking for merge
commits will help understand how the git organizes history.

In the following, we say that commit X is "reachable" from commit Y
if commit X is an ancestor of commit Y. Equivalently, you could say
that Y is a descendant of X, or that there is a chain of parents
leading from commit Y to commit X.

Understanding history: History diagrams

We will sometimes represent git history using diagrams like the one
below. Commits are shown as "o", and the links between them with
lines drawn with - / and \. Time goes left to right:

o--o--o <-- Branch A
/
o--o--o <-- master
\
o--o--o <-- Branch B

If we need to talk about a particular commit, the character "o" may
be replaced with another letter or number.

Understanding history: What is a branch?

When we need to be precise, we will use the word "branch" to mean a line
of development, and "branch head" (or just "head") to mean a reference
to the most recent commit on a branch. In the example above, the branch
head named "A" is a pointer to one particular commit, but we refer to
the line of three commits leading up to that point as all being part of
"branch A".

However, when no confusion will result, we often just use the term
"branch" both for branches and for branch heads.

Manipulating branches

Creating, deleting, and modifying branches is quick and easy; here’s
a summary of the commands:

git branch

list all branches

git branch <branch>

create a new branch named <branch>, referencing the same
point in history as the current branch

git branch <branch> <start-point>

create a new branch named <branch>, referencing
<start-point>, which may be specified any way you like,
including using a branch name or a tag name

git branch -d <branch>

delete the branch <branch>; if the branch you are deleting
points to a commit which is not reachable from the current
branch, this command will fail with a warning.

git branch -D <branch>

even if the branch points to a commit not reachable
from the current branch, you may know that that commit
is still reachable from some other branch or tag. In that
case it is safe to use this command to force git to delete
the branch.

git checkout <branch>

make the current branch <branch>, updating the working
directory to reflect the version referenced by <branch>

git checkout -b <new> <start-point>

create a new branch <new> referencing <start-point>, and
check it out.

The special symbol "HEAD" can always be used to refer to the current
branch. In fact, git uses a file named "HEAD" in the .git directory to
remember which branch is current:

$ cat .git/HEAD
ref: refs/heads/master

Examining an old version without creating a new branch

The git checkout command normally expects a branch head, but will also
accept an arbitrary commit; for example, you can check out the commit
referenced by a tag:

$ git checkout v2.6.17
Note: moving to "v2.6.17" which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
git checkout -b <new_branch_name>
HEAD is now at 427abfa... Linux v2.6.17

The HEAD then refers to the SHA-1 of the commit instead of to a branch,
and git branch shows that you are no longer on a branch:

This is an easy way to check out a particular version without having to
make up a name for the new branch. You can still create a new branch
(or tag) for this version later if you decide to.

Examining branches from a remote repository

The "master" branch that was created at the time you cloned is a copy
of the HEAD in the repository that you cloned from. That repository
may also have had other branches, though, and your local repository
keeps branches which track each of those remote branches, called
remote-tracking branches, which you
can view using the "-r" option to git-branch(1):

In this example, "origin" is called a remote repository, or "remote"
for short. The branches of this repository are called "remote
branches" from our point of view. The remote-tracking branches listed
above were created based on the remote branches at clone time and will
be updated by "git fetch" (hence "git pull") and "git push". See
the section called “Updating a repository with git fetch” for details.

You might want to build on one of these remote-tracking branches
on a branch of your own, just as you would for a tag:

$ git checkout -b my-todo-copy origin/todo

You can also check out "origin/todo" directly to examine it or
write a one-off patch. See detached head.

Note that the name "origin" is just the name that git uses by default
to refer to the repository that you cloned from.

Naming branches, tags, and other references

Branches, remote-tracking branches, and tags are all references to
commits. All references are named with a slash-separated path name
starting with "refs"; the names we’ve been using so far are actually
shorthand:

The branch "test" is short for "refs/heads/test".

The tag "v2.6.18" is short for "refs/tags/v2.6.18".

"origin/master" is short for "refs/remotes/origin/master".

The full name is occasionally useful if, for example, there ever
exists a tag and a branch with the same name.

(Newly created refs are actually stored in the .git/refs directory,
under the path given by their name. However, for efficiency reasons
they may also be packed together in a single file; see
git-pack-refs(1)).

As another useful shortcut, the "HEAD" of a repository can be referred
to just using the name of that repository. So, for example, "origin"
is usually a shortcut for the HEAD branch in the repository "origin".

For the complete list of paths which git checks for references, and
the order it uses to decide which to choose when there are multiple
references with the same shorthand name, see the "SPECIFYING
REVISIONS" section of gitrevisions(7).

Updating a repository with git fetch

Eventually the developer cloned from will do additional work in her
repository, creating new commits and advancing the branches to point
at the new commits.

The command "git fetch", with no arguments, will update all of the
remote-tracking branches to the latest version found in her
repository. It will not touch any of your own branches—not even the
"master" branch that was created for you on clone.

Fetching branches from other repositories

You can also track branches from repositories other than the one you
cloned from, using git-remote(1):

This is what causes git to track the remote’s branches; you may modify
or delete these configuration options by editing .git/config with a
text editor. (See the "CONFIGURATION FILE" section of
git-config(1) for details.)

Git is best thought of as a tool for storing the history of a
collection of files. It does this by storing compressed snapshots of
the contents of a file hierarchy, together with "commits" which show
the relationships between these snapshots.

Git provides extremely flexible and fast tools for exploring the
history of a project.

We start with one specialized tool that is useful for finding the
commit that introduced a bug into a project.

How to use bisect to find a regression

Suppose version 2.6.18 of your project worked, but the version at
"master" crashes. Sometimes the best way to find the cause of such a
regression is to perform a brute-force search through the project’s
history to find the particular commit that caused the problem. The
git-bisect(1) command can help you do this:

If you run "git branch" at this point, you’ll see that git has
temporarily moved you in "(no branch)". HEAD is now detached from any
branch and points directly to a commit (with commit id 65934…) that
is reachable from "master" but not from v2.6.18. Compile and test it,
and see whether it crashes. Assume it does crash. Then:

checks out an older version. Continue like this, telling git at each
stage whether the version it gives you is good or bad, and notice
that the number of revisions left to test is cut approximately in
half each time.

After about 13 tests (in this case), it will output the commit id of
the guilty commit. You can then examine the commit with
git-show(1), find out who wrote it, and mail them your bug
report with the commit id. Finally, run

$ git bisect reset

to return you to the branch you were on before.

Note that the version which git bisect checks out for you at each
point is just a suggestion, and you’re free to try a different
version if you think it would be a good idea. For example,
occasionally you may land on a commit that broke something unrelated;
run

$ git bisect visualize

which will run gitk and label the commit it chose with a marker that
says "bisect". Choose a safe-looking commit nearby, note its commit
id, and check it out with:

$ git reset --hard fb47ddb2db...

then test, run "bisect good" or "bisect bad" as appropriate, and
continue.

Instead of "git bisect visualize" and then "git reset --hard
fb47ddb2db…", you might just want to tell git that you want to skip
the current commit:

$ git bisect skip

In this case, though, git may not eventually be able to tell the first
bad one between some first skipped commits and a later bad commit.

There are also ways to automate the bisecting process if you have a
test script that can tell a good from a bad commit. See
git-bisect(1) for more information about this and other "git
bisect" features.

Naming commits

We have seen several ways of naming commits already:

40-hexdigit object name

branch name: refers to the commit at the head of the given
branch

tag name: refers to the commit pointed to by the given tag
(we’ve seen branches and tags are special cases of
references).

HEAD: refers to the head of the current branch

There are many more; see the "SPECIFYING REVISIONS" section of the
gitrevisions(7) man page for the complete list of ways to
name revisions. Some examples:

$ git show fb47ddb2 # the first few characters of the object name
# are usually enough to specify it uniquely
$ git show HEAD^ # the parent of the HEAD commit
$ git show HEAD^^ # the grandparent
$ git show HEAD~4 # the great-great-grandparent

Recall that merge commits may have more than one parent; by default,
^ and ~ follow the first parent listed in the commit, but you can
also choose:

$ git show HEAD^1 # show the first parent of HEAD
$ git show HEAD^2 # show the second parent of HEAD

In addition to HEAD, there are several other special names for
commits:

Merges (to be discussed later), as well as operations such as
git reset, which change the currently checked-out commit, generally
set ORIG_HEAD to the value HEAD had before the current operation.

The git fetch operation always stores the head of the last fetched
branch in FETCH_HEAD. For example, if you run git fetch without
specifying a local branch as the target of the operation

$ git fetch git://example.com/proj.git theirbranch

the fetched commits will still be available from FETCH_HEAD.

When we discuss merges we’ll also see the special name MERGE_HEAD,
which refers to the other branch that we’re merging in to the current
branch.

The git-rev-parse(1) command is a low-level command that is
occasionally useful for translating some name for a commit to the object
name for that commit:

$ git rev-parse origin
e05db0fd4f31dde7005f075a84f96b360d05984b

Creating tags

We can also create a tag to refer to a particular commit; after
running

$ git tag stable-1 1b2e1d63ff

You can use stable-1 to refer to the commit 1b2e1d63ff.

This creates a "lightweight" tag. If you would also like to include a
comment with the tag, and possibly sign it cryptographically, then you
should create a tag object instead; see the git-tag(1) man page
for details.

Browsing revisions

The git-log(1) command can show lists of commits. On its
own, it shows all commits reachable from the parent commit; but you
can also make more specific requests:

And of course you can combine all of these; the following finds
commits since v2.5 which touch the Makefile or any file under fs:

$ git log v2.5.. Makefile fs/

You can also ask git log to show patches:

$ git log -p

See the "--pretty" option in the git-log(1) man page for more
display options.

Note that git log starts with the most recent commit and works
backwards through the parents; however, since git history can contain
multiple independent lines of development, the particular order that
commits are listed in may be somewhat arbitrary.

will generate a file with a patch for each commit reachable from test
but not from master.

Viewing old file versions

You can always view an old version of a file by just checking out the
correct revision first. But sometimes it is more convenient to be
able to view an old version of a single file without checking
anything out; this command does that:

$ git show v2.5:fs/locks.c

Before the colon may be anything that names a commit, and after it
may be any path to a file tracked by git.

Examples

Counting the number of commits on a branch

Suppose you want to know how many commits you’ve made on "mybranch"
since it diverged from "origin":

$ git log --pretty=oneline origin..mybranch | wc -l

Alternatively, you may often see this sort of thing done with the
lower-level command git-rev-list(1), which just lists the SHA-1’s
of all the given commits:

$ git rev-list origin..mybranch | wc -l

Check whether two branches point at the same history

Suppose you want to check whether two branches point at the same point
in history.

$ git diff origin..master

will tell you whether the contents of the project are the same at the
two branches; in theory, however, it’s possible that the same project
contents could have been arrived at by two different historical
routes. You could compare the object names:

The merge-base command finds a common ancestor of the given commits,
and always returns one or the other in the case where one is a
descendant of the other; so the above output shows that e05db0fd
actually is an ancestor of v1.5.0-rc1.

Alternatively, note that

$ git log v1.5.0-rc1..e05db0fd

will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
because it outputs only commits that are not reachable from v1.5.0-rc1.

As yet another alternative, the git-show-branch(1) command lists
the commits reachable from its arguments with a display on the left-hand
side that indicates which arguments that commit is reachable from. So,
you can run something like

How to make a commit

Making some changes to the working directory using your
favorite editor.

Telling git about your changes.

Creating the commit using the content you told git about
in step 2.

In practice, you can interleave and repeat steps 1 and 2 as many
times as you want: in order to keep track of what you want committed
at step 3, git maintains a snapshot of the tree’s contents in a
special staging area called "the index."

At the beginning, the content of the index will be identical to
that of the HEAD. The command "git diff --cached", which shows
the difference between the HEAD and the index, should therefore
produce no output at that point.

Modifying the index is easy:

To update the index with the new contents of a modified file, use

$ git add path/to/file

To add the contents of a new file to the index, use

$ git add path/to/file

To remove a file from the index and from the working tree,

$ git rm path/to/file

After each step you can verify that

$ git diff --cached

always shows the difference between the HEAD and the index file—this
is what you’d commit if you created the commit now—and that

$ git diff

shows the difference between the working tree and the index file.

Note that "git add" always adds just the current contents of a file
to the index; further changes to the same file will be ignored unless
you run git add on the file again.

When you’re ready, just run

$ git commit

and git will prompt you for a commit message and then create the new
commit. Check to make sure it looks like what you expected with

$ git show

As a special shortcut,

$ git commit -a

will update the index with any files that you’ve modified or removed
and create a commit, all in one step.

A number of commands are useful for keeping track of what you’re
about to commit:

$ git diff --cached # difference between HEAD and the index; what
# would be committed if you ran "commit" now.
$ git diff # difference between the index file and your
# working directory; changes that would not
# be included if you ran "commit" now.
$ git diff HEAD # difference between HEAD and working tree; what
# would be committed if you ran "commit -a" now.
$ git status # a brief per-file summary of the above.

You can also use git-gui(1) to create commits, view changes in
the index and the working tree files, and individually select diff hunks
for inclusion in the index (by right-clicking on the diff hunk and
choosing "Stage Hunk For Commit").

Creating good commit messages

Though not required, it’s a good idea to begin the commit message
with a single short (less than 50 character) line summarizing the
change, followed by a blank line and then a more thorough
description. Tools that turn commits into email, for example, use
the first line on the Subject line and the rest of the commit in the
body.

Ignoring files

A project will often generate files that you do not want to track with git.
This typically includes files generated by a build process or temporary
backup files made by your editor. Of course, not tracking files with git
is just a matter of not calling git add on them. But it quickly becomes
annoying to have these untracked files lying around; e.g. they make
git add . practically useless, and they keep showing up in the output of
git status.

You can tell git to ignore certain files by creating a file called .gitignore
in the top level of your working directory, with contents such as:

See gitignore(5) for a detailed explanation of the syntax. You can
also place .gitignore files in other directories in your working tree, and they
will apply to those directories and their subdirectories. The .gitignore
files can be added to your repository like any other files (just run git add
.gitignore and git commit, as usual), which is convenient when the exclude
patterns (such as patterns matching build output files) would also make sense
for other users who clone your repository.

If you wish the exclude patterns to affect only certain repositories
(instead of every repository for a given project), you may instead put
them in a file in your repository named .git/info/exclude, or in any file
specified by the core.excludesfile configuration variable. Some git
commands can also take exclude patterns directly on the command line.
See gitignore(5) for the details.

How to merge

You can rejoin two diverging branches of development using
git-merge(1):

$ git merge branchname

merges the development in the branch "branchname" into the current
branch.

A merge is made by combining the changes made in "branchname" and the
changes made up to the latest commit in your current branch since
their histories forked. The work tree is overwritten by the result of
the merge when this combining is done cleanly, or overwritten by a
half-merged results when this combining results in conflicts.
Therefore, if you have uncommitted changes touching the same files as
the ones impacted by the merge, Git will refuse to proceed. Most of
the time, you will want to commit your changes before you can merge,
and if you don’t, then git-stash(1) can take these changes
away while you’re doing the merge, and reapply them afterwards.

If the changes are independent enough, Git will automatically complete
the merge and commit the result (or reuse an existing commit in case
of fast-forward, see below). On the other hand,
if there are conflicts—for example, if the same file is
modified in two different ways in the remote branch and the local
branch—then you are warned; the output may look something like this:

Conflict markers are left in the problematic files, and after
you resolve the conflicts manually, you can update the index
with the contents and run git commit, as you normally would when
creating a new file.

If you examine the resulting commit using gitk, you will see that it
has two parents, one pointing to the top of the current branch, and
one to the top of the other branch.

Resolving a merge

When a merge isn’t resolved automatically, git leaves the index and
the working tree in a special state that gives you all the
information you need to help resolve the merge.

Files with conflicts are marked specially in the index, so until you
resolve the problem and update the index, git-commit(1) will
fail:

$ git commit
file.txt: needs merge

Also, git-status(1) will list those files as "unmerged", and the
files with conflicts will have conflict markers added, like this:

All you need to do is edit the files to resolve the conflicts, and then

$ git add file.txt
$ git commit

Note that the commit message will already be filled in for you with
some information about the merge. Normally you can just use this
default message unchanged, but you may add additional commentary of
your own if desired.

The above is all you need to know to resolve a simple merge. But git
also provides more information to help resolve conflicts:

Getting conflict-resolution help during a merge

All of the changes that git was able to merge automatically are
already added to the index file, so git-diff(1) shows only
the conflicts. It uses an unusual syntax:

Recall that the commit which will be committed after we resolve this
conflict will have two parents instead of the usual one: one parent
will be HEAD, the tip of the current branch; the other will be the
tip of the other branch, which is stored temporarily in MERGE_HEAD.

During the merge, the index holds three versions of each file. Each of
these three "file stages" represents a different version of the file:

$ git show :1:file.txt # the file in a common ancestor of both branches
$ git show :2:file.txt # the version from HEAD.
$ git show :3:file.txt # the version from MERGE_HEAD.

When you ask git-diff(1) to show the conflicts, it runs a
three-way diff between the conflicted merge results in the work tree with
stages 2 and 3 to show only hunks whose contents come from both sides,
mixed (in other words, when a hunk’s merge results come only from stage 2,
that part is not conflicting and is not shown. Same for stage 3).

The diff above shows the differences between the working-tree version of
file.txt and the stage 2 and stage 3 versions. So instead of preceding
each line by a single "+" or "-", it now uses two columns: the first
column is used for differences between the first parent and the working
directory copy, and the second for differences between the second parent
and the working directory copy. (See the "COMBINED DIFF FORMAT" section
of git-diff-files(1) for a details of the format.)

After resolving the conflict in the obvious way (but before updating the
index), the diff will look like:

These will display all commits which exist only on HEAD or on
MERGE_HEAD, and which touch an unmerged file.

You may also use git-mergetool(1), which lets you merge the
unmerged files using external tools such as Emacs or kdiff3.

Each time you resolve the conflicts in a file and update the index:

$ git add file.txt

the different stages of that file will be "collapsed", after which
git diff will (by default) no longer show diffs for that file.

Undoing a merge

If you get stuck and decide to just give up and throw the whole mess
away, you can always return to the pre-merge state with

$ git reset --hard HEAD

Or, if you’ve already committed the merge that you want to throw away,

$ git reset --hard ORIG_HEAD

However, this last command can be dangerous in some cases—never
throw away a commit you have already committed if that commit may
itself have been merged into another branch, as doing so may confuse
further merges.

Fast-forward merges

There is one special case not mentioned above, which is treated
differently. Normally, a merge results in a merge commit, with two
parents, one pointing at each of the two lines of development that
were merged.

However, if the current branch is a descendant of the other—so every
commit present in the one is already contained in the other—then git
just performs a "fast-forward"; the head of the current branch is moved
forward to point at the head of the merged-in branch, without any new
commits being created.

Fixing mistakes

If you’ve messed up the working tree, but haven’t yet committed your
mistake, you can return the entire working tree to the last committed
state with

$ git reset --hard HEAD

If you make a commit that you later wish you hadn’t, there are two
fundamentally different ways to fix the problem:

You can create a new commit that undoes whatever was done
by the old commit. This is the correct thing if your
mistake has already been made public.

You can go back and modify the old commit. You should
never do this if you have already made the history public;
git does not normally expect the "history" of a project to
change, and cannot correctly perform repeated merges from
a branch that has had its history changed.

Fixing a mistake with a new commit

Creating a new commit that reverts an earlier change is very easy;
just pass the git-revert(1) command a reference to the bad
commit; for example, to revert the most recent commit:

$ git revert HEAD

This will create a new commit which undoes the change in HEAD. You
will be given a chance to edit the commit message for the new commit.

You can also revert an earlier change, for example, the next-to-last:

$ git revert HEAD^

In this case git will attempt to undo the old change while leaving
intact any changes made since then. If more recent changes overlap
with the changes to be reverted, then you will be asked to fix
conflicts manually, just as in the case of resolving a merge.

Fixing a mistake by rewriting history

If the problematic commit is the most recent commit, and you have not
yet made that commit public, then you may just
destroy it using git reset.

Alternatively, you
can edit the working directory and update the index to fix your
mistake, just as if you were going to create a new commit, then run

$ git commit --amend

which will replace the old commit by a new commit incorporating your
changes, giving you a chance to edit the old commit message first.

Again, you should never do this to a commit that may already have
been merged into another branch; use git-revert(1) instead in
that case.

It is also possible to replace commits further back in the history, but
this is an advanced topic to be left for
another chapter.

Checking out an old version of a file

In the process of undoing a previous bad change, you may find it
useful to check out an older version of a particular file using
git-checkout(1). We’ve used git checkout before to switch
branches, but it has quite different behavior if it is given a path
name: the command

$ git checkout HEAD^ path/to/file

replaces path/to/file by the contents it had in the commit HEAD^, and
also updates the index to match. It does not change branches.

If you just want to look at an old version of the file, without
modifying the working directory, you can do that with
git-show(1):

$ git show HEAD^:path/to/file

which will display the given version of the file.

Temporarily setting aside work in progress

While you are in the middle of working on something complicated, you
find an unrelated but obvious and trivial bug. You would like to fix it
before continuing. You can use git-stash(1) to save the current
state of your work, and after fixing the bug (or, optionally after doing
so on a different branch and then coming back), unstash the
work-in-progress changes.

$ git stash save "work in progress for foo feature"

This command will save your changes away to the stash, and
reset your working tree and the index to match the tip of your
current branch. Then you can make your fix as usual.

... edit and test ...
$ git commit -a -m "blorpl: typofix"

After that, you can go back to what you were working on with
git stash pop:

$ git stash pop

Ensuring good performance

On large repositories, git depends on compression to keep the history
information from taking up too much space on disk or in memory.

This compression is not performed automatically. Therefore you
should occasionally run git-gc(1):

$ git gc

to recompress the archive. This can be very time-consuming, so
you may prefer to run git gc when you are not doing other work.

Ensuring reliability

Checking the repository for corruption

The git-fsck(1) command runs a number of self-consistency checks
on the repository, and reports on any problems. This may take some
time. The most common warning by far is about "dangling" objects:

Dangling objects are not a problem. At worst they may take up a little
extra disk space. They can sometimes provide a last-resort method for
recovering lost work—see the section called “Dangling objects” for details.

Recovering lost changes

Reflogs

Say you modify a branch with git-reset(1) --hard, and then
realize that the branch was the only reference you had to that point in
history.

Fortunately, git also keeps a log, called a "reflog", of all the
previous values of each branch. So in this case you can still find the
old history using, for example,

$ git log master@{1}

This lists the commits reachable from the previous version of the
"master" branch head. This syntax can be used with any git command
that accepts a commit, not just with git log. Some other examples:

will show what HEAD pointed to one week ago, not what the current branch
pointed to one week ago. This allows you to see the history of what
you’ve checked out.

The reflogs are kept by default for 30 days, after which they may be
pruned. See git-reflog(1) and git-gc(1) to learn
how to control this pruning, and see the "SPECIFYING REVISIONS"
section of gitrevisions(7) for details.

Note that the reflog history is very different from normal git history.
While normal history is shared by every repository that works on the
same project, the reflog history is not shared: it tells you only about
how the branches in your local repository have changed over time.

Examining dangling objects

In some situations the reflog may not be able to save you. For example,
suppose you delete a branch, then realize you need the history it
contained. The reflog is also deleted; however, if you have not yet
pruned the repository, then you may still be able to find the lost
commits in the dangling objects that git fsck reports. See
the section called “Dangling objects” for the details.

which does what it sounds like: it says that you want to see the commit
history that is described by the dangling commit(s), but not the
history that is described by all your existing branches and tags. Thus
you get exactly the history reachable from that commit that is lost.
(And notice that it might not be just one commit: we only report the
"tip of the line" as being dangling, but there might be a whole deep
and complex commit history that was dropped.)

If you decide you want the history back, you can always create a new
reference pointing to it, for example, a new branch:

$ git branch recovered-branch 7281251ddd

Other types of dangling objects (blobs and trees) are also possible, and
dangling objects can arise in other situations.

However, the git-pull(1) command provides a way to do this in
one step:

$ git pull origin master

In fact, if you have "master" checked out, then this branch has been
configured by "git clone" to get changes from the HEAD branch of the
origin repository. So often you can
accomplish the above with just a simple

$ git pull

This command will fetch changes from the remote branches to your
remote-tracking branches origin/*, and merge the default branch into
the current branch.

More generally, a branch that is created from a remote-tracking branch
will pull
by default from that branch. See the descriptions of the
branch.<name>.remote and branch.<name>.merge options in
git-config(1), and the discussion of the --track option in
git-checkout(1), to learn how to control these defaults.

In addition to saving you keystrokes, "git pull" also helps you by
producing a default commit message documenting the branch and
repository that you pulled from.

(But note that no such commit will be created in the case of a
fast-forward; instead, your branch will just be
updated to point to the latest commit from the upstream branch.)

The git pull command can also be given "." as the "remote" repository,
in which case it just merges in a branch from the current repository; so
the commands

$ git pull . branch
$ git merge branch

are roughly equivalent. The former is actually very commonly used.

Submitting patches to a project

If you just have a few changes, the simplest way to submit them may
just be to send them as patches in email:

will produce a numbered series of files in the current directory, one
for each patch in the current branch but not in origin/HEAD.

You can then import these into your mail client and send them by
hand. However, if you have a lot to send at once, you may prefer to
use the git-send-email(1) script to automate the process.
Consult the mailing list for your project first to determine how they
prefer such patches be handled.

Importing patches to a project

Git also provides a tool called git-am(1) (am stands for
"apply mailbox"), for importing such an emailed series of patches.
Just save all of the patch-containing messages, in order, into a
single mailbox file, say "patches.mbox", then run

$ git am -3 patches.mbox

Git will apply each patch in order; if any conflicts are found, it
will stop, and you can fix the conflicts as described in
"Resolving a merge". (The "-3" option tells
git to perform a merge; if you would prefer it just to abort and
leave your tree and index untouched, you may omit that option.)

Once the index is updated with the results of the conflict
resolution, instead of creating a new commit, just run

$ git am --resolved

and git will create the commit for you and continue applying the
remaining patches from the mailbox.

The final result will be a series of commits, one for each patch in
the original mailbox, with authorship and commit log message each
taken from the message containing each patch.

Public git repositories

Another way to submit changes to a project is to tell the maintainer
of that project to pull the changes from your repository using
git-pull(1). In the section "Getting updates with git pull" we described this as a way to get
updates from the "main" repository, but it works just as well in the
other direction.

If you and the maintainer both have accounts on the same machine, then
you can just pull changes from each other’s repositories directly;
commands that accept repository URLs as arguments will also accept a
local directory name:

$ git clone /path/to/repository
$ git pull /path/to/other/repository

or an ssh URL:

$ git clone ssh://yourhost/~you/repository

For projects with few developers, or for synchronizing a few private
repositories, this may be all you need.

However, the more common way to do this is to maintain a separate public
repository (usually on a different host) for others to pull changes
from. This is usually more convenient, and allows you to cleanly
separate private work in progress from publicly visible work.

You will continue to do your day-to-day work in your personal
repository, but periodically "push" changes from your personal
repository into your public repository, allowing other developers to
pull from that repository. So the flow of changes, in a situation
where there is one other developer with a public repository, looks
like this:

The resulting directory proj.git contains a "bare" git repository—it is
just the contents of the ".git" directory, without any files checked out
around it.

Next, copy proj.git to the server where you plan to host the
public repository. You can use scp, rsync, or whatever is most
convenient.

Exporting a git repository via the git protocol

This is the preferred method.

If someone else administers the server, they should tell you what
directory to put the repository in, and what git:// URL it will appear
at. You can then skip to the section
"Pushing changes to a public repository", below.

Otherwise, all you need to do is start git-daemon(1); it will
listen on port 9418. By default, it will allow access to any directory
that looks like a git directory and contains the magic file
git-daemon-export-ok. Passing some directory paths as git daemon
arguments will further restrict the exports to those paths.

You can also run git daemon as an inetd service; see the
git-daemon(1) man page for details. (See especially the
examples section.)

Exporting a git repository via http

The git protocol gives better performance and reliability, but on a
host with a web server set up, http exports may be simpler to set up.

All you need to do is place the newly created bare git repository in
a directory that is exported by the web server, and make some
adjustments to give web clients some extra information they need:

Pushing changes to a public repository

Note that the two techniques outlined above (exporting via
http or git) allow other
maintainers to fetch your latest changes, but they do not allow write
access, which you will need to update the public repository with the
latest changes created in your private repository.

The simplest way to do this is using git-push(1) and ssh; to
update the remote branch named "master" with the latest state of your
branch named "master", run

$ git push ssh://yourserver.com/~you/proj.git master:master

or just

$ git push ssh://yourserver.com/~you/proj.git master

As with git fetch, git push will complain if this does not result in a
fast-forward; see the following section for details on
handling this case.

Note that the target of a "push" is normally a
bare repository. You can also push to a
repository that has a checked-out working tree, but the working tree
will not be updated by the push. This may lead to unexpected results if
the branch you push to is the currently checked-out branch!

As with git fetch, you may also set up configuration options to
save typing; so, for example, after

You may force git push to perform the update anyway by preceding the
branch name with a plus sign:

$ git push ssh://yourserver.com/~you/proj.git +master

Normally whenever a branch head in a public repository is modified, it
is modified to point to a descendant of the commit that it pointed to
before. By forcing a push in this situation, you break that convention.
(See the section called “Problems with rewriting history”.)

Nevertheless, this is a common practice for people that need a simple
way to publish a work-in-progress patch series, and it is an acceptable
compromise as long as you warn other developers that this is how you
intend to manage the branch.

It’s also possible for a push to fail in this way when other people have
the right to push to the same repository. In that case, the correct
solution is to retry the push after first updating your work: either by a
pull, or by a fetch followed by a rebase; see the
next section and
gitcvs-migration(7) for more.

Setting up a shared repository

Another way to collaborate is by using a model similar to that
commonly used in CVS, where several developers with special rights
all push to and pull from a single shared repository. See
gitcvs-migration(7) for instructions on how to
set this up.

However, while there is nothing wrong with git’s support for shared
repositories, this mode of operation is not generally recommended,
simply because the mode of collaboration that git supports—by
exchanging patches and pulling from public repositories—has so many
advantages over the central shared repository:

Git’s ability to quickly import and merge patches allows a
single maintainer to process incoming changes even at very
high rates. And when that becomes too much, git pull provides
an easy way for that maintainer to delegate this job to other
maintainers while still allowing optional review of incoming
changes.

Since every developer’s repository has the same complete copy
of the project history, no repository is special, and it is
trivial for another developer to take over maintenance of a
project, either by mutual agreement, or because a maintainer
becomes unresponsive or difficult to work with.

The lack of a central group of "committers" means there is
less need for formal decisions about who is "in" and who is
"out".

Allowing web browsing of a repository

The gitweb cgi script provides users an easy way to browse your
project’s files and history without having to install git; see the file
gitweb/INSTALL in the git source tree for instructions on setting it up.

Examples

Maintaining topic branches for a Linux subsystem maintainer

This describes how Tony Luck uses git in his role as maintainer of the
IA64 architecture for the Linux kernel.

He uses two public branches:

A "test" tree into which patches are initially placed so that they
can get some exposure when integrated with other ongoing development.
This tree is available to Andrew for pulling into -mm whenever he
wants.

A "release" tree into which tested patches are moved for final sanity
checking, and as a vehicle to send them upstream to Linus (by sending
him a "please pull" request.)

He also uses a set of temporary branches ("topic branches"), each
containing a logical grouping of patches.

To set this up, first create your work tree by cloning Linus’s public
tree:

Now create the branches in which you are going to work; these start out
at the current tip of origin/master branch, and should be set up (using
the --track option to git-branch(1)) to merge changes in from
Linus by default.

Important note! If you have any local changes in these branches, then
this merge will create a commit object in the history (with no local
changes git will simply do a "fast-forward" merge). Many people dislike
the "noise" that this creates in the Linux history, so you should avoid
doing this capriciously in the "release" branch, as these noisy commits
will become part of the permanent history when you ask Linus to pull
from the release branch.

Now to apply some patches from the community. Think of a short
snappy name for a branch to hold this patch (or related group of
patches), and create a new branch from a recent stable tag of
Linus’s branch. Picking a stable base for your branch will:
1) help you: by avoiding inclusion of unrelated and perhaps lightly
tested changes
2) help future bug hunters that use "git bisect" to find problems

$ git checkout -b speed-up-spinlocks v2.6.35

Now you apply the patch(es), run some tests, and commit the change(s). If
the patch is a multi-part series, then you should apply each as a separate
commit to this branch.

$ ... patch ... test ... commit [ ... patch ... test ... commit ]*

When you are happy with the state of this change, you can pull it into the
"test" branch in preparation to make it public:

$ git checkout test && git pull . speed-up-spinlocks

It is unlikely that you would have any conflicts here … but you might if you
spent a while on this step and had also pulled new versions from upstream.

Some time later when enough time has passed and testing done, you can pull the
same branch into the "release" tree ready to go upstream. This is where you
see the value of keeping each patch (or patch series) in its own branch. It
means that the patches can be moved into the "release" tree in any order.

$ git checkout release && git pull . speed-up-spinlocks

After a while, you will have a number of branches, and despite the
well chosen names you picked for each of them, you may forget what
they are for, or what status they are in. To get a reminder of what
changes are in a specific branch, use:

$ git log linux..branchname | git shortlog

To see whether it has already been merged into the test or release branches,
use:

$ git log test..branchname

or

$ git log release..branchname

(If this branch has not yet been merged, you will see some log entries.
If it has been merged, then there will be no output.)

Once a patch completes the great cycle (moving from test to release,
then pulled by Linus, and finally coming back into your local
"origin/master" branch), the branch for this change is no longer needed.
You detect this when the output from:

$ git log origin..branchname

is empty. At this point the branch can be deleted:

$ git branch -d branchname

Some changes are so trivial that it is not necessary to create a separate
branch and then merge into each of the test and release branches. For
these changes, just apply directly to the "release" branch, and then
merge that into the "test" branch.

To create diffstat and shortlog summaries of changes to include in a "please
pull" request to Linus you can use:

$ git diff --stat origin..release

and

$ git log -p origin..release | git shortlog

Here are some of the scripts that simplify all this even further.

==== update script ====
# Update a branch in my GIT tree. If the branch to be updated
# is origin, then pull from kernel.org. Otherwise merge
# origin/master branch into test|release branch

Normally commits are only added to a project, never taken away or
replaced. Git is designed with this assumption, and violating it will
cause git’s merge machinery (for example) to do the wrong thing.

However, there is a situation in which it can be useful to violate this
assumption.

Creating the perfect patch series

Suppose you are a contributor to a large project, and you want to add a
complicated feature, and to present it to the other developers in a way
that makes it easy for them to read your changes, verify that they are
correct, and understand why you made each change.

If you present all of your changes as a single patch (or commit), they
may find that it is too much to digest all at once.

If you present them with the entire history of your work, complete with
mistakes, corrections, and dead ends, they may be overwhelmed.

So the ideal is usually to produce a series of patches such that:

Each patch can be applied in order.

Each patch includes a single logical change, together with a
message explaining the change.

No patch introduces a regression: after applying any initial
part of the series, the resulting project still compiles and
works, and has no bugs that it didn’t have before.

The complete series produces the same end result as your own
(probably much messier!) development process did.

We will introduce some tools that can help you do this, explain how to
use them, and then explain some of the problems that can arise because
you are rewriting history.

Keeping a patch series up to date using git rebase

Suppose that you create a branch "mywork" on a remote-tracking branch
"origin", and create some commits on top of it:

You have performed no merges into mywork, so it is just a simple linear
sequence of patches on top of "origin":

o--o--O <-- origin
\
a--b--c <-- mywork

Some more interesting work has been done in the upstream project, and
"origin" has advanced:

o--o--O--o--o--o <-- origin
\
a--b--c <-- mywork

At this point, you could use "pull" to merge your changes back in;
the result would create a new merge commit, like this:

o--o--O--o--o--o <-- origin
\ \
a--b--c--m <-- mywork

However, if you prefer to keep the history in mywork a simple series of
commits without any merges, you may instead choose to use
git-rebase(1):

$ git checkout mywork
$ git rebase origin

This will remove each of your commits from mywork, temporarily saving
them as patches (in a directory named ".git/rebase-apply"), update mywork to
point at the latest version of origin, then apply each of the saved
patches to the new mywork. The result will look like:

o--o--O--o--o--o <-- origin
\
a'--b'--c' <-- mywork

In the process, it may discover conflicts. In that case it will stop
and allow you to fix the conflicts; after fixing conflicts, use git add
to update the index with those contents, and then, instead of
running git commit, just run

$ git rebase --continue

and git will continue applying the rest of the patches.

At any point you may use the --abort option to abort this process and
return mywork to the state it had before you started the rebase:

When you’re done, you’ll be left with mywork checked out, with the top
patches on mywork reapplied on top of your modified commit. You can
then clean up with

$ git tag -d bad

Note that the immutable nature of git history means that you haven’t really
"modified" existing commits; instead, you have replaced the old commits with
new commits having new object names.

Reordering or selecting from a patch series

Given one existing commit, the git-cherry-pick(1) command
allows you to apply the change introduced by that commit and create a
new commit that records it. So, for example, if "mywork" points to a
series of patches on top of "origin", you might do something like:

$ git checkout -b mywork-new origin
$ gitk origin..mywork &

and browse through the list of patches in the mywork branch using gitk,
applying them (possibly in a different order) to mywork-new using
cherry-pick, and possibly modifying them as you go using git commit --amend.
The git-gui(1) command may also help as it allows you to
individually select diff hunks for inclusion in the index (by
right-clicking on the diff hunk and choosing "Stage Hunk for Commit").

Another technique is to use git format-patch to create a series of
patches, then reset the state to before the patches:

$ git format-patch origin
$ git reset --hard origin

Then modify, reorder, or eliminate patches as preferred before applying
them again with git-am(1).

Other tools

There are numerous other tools, such as StGit, which exist for the
purpose of maintaining a patch series. These are outside of the scope of
this manual.

Problems with rewriting history

The primary problem with rewriting the history of a branch has to do
with merging. Suppose somebody fetches your branch and merges it into
their branch, with a result something like this:

o--o--O--o--o--o <-- origin
\ \
t--t--t--m <-- their branch:

Then suppose you modify the last three commits:

o--o--o <-- new head of origin
/
o--o--O--o--o--o <-- old head of origin

If we examined all this history together in one repository, it will
look like:

o--o--o <-- new head of origin
/
o--o--O--o--o--o <-- old head of origin
\ \
t--t--t--m <-- their branch:

Git has no way of knowing that the new head is an updated version of
the old head; it treats this situation exactly the same as it would if
two developers had independently done the work on the old and new heads
in parallel. At this point, if someone attempts to merge the new head
in to their branch, git will attempt to merge together the two (old and
new) lines of development, instead of trying to replace the old by the
new. The results are likely to be unexpected.

You may still choose to publish branches whose history is rewritten,
and it may be useful for others to be able to fetch those branches in
order to examine or test them, but they should not attempt to pull such
branches into their own work.

For true distributed development that supports proper merging,
published branches should never be rewritten.

Why bisecting merge commits can be harder than bisecting linear history

The git-bisect(1) command correctly handles history that
includes merge commits. However, when the commit that it finds is a
merge commit, the user may need to work harder than usual to figure out
why that commit introduced a problem.

Imagine this history:

---Z---o---X---...---o---A---C---D
\ /
o---o---Y---...---o---B

Suppose that on the upper line of development, the meaning of one
of the functions that exists at Z is changed at commit X. The
commits from Z leading to A change both the function’s
implementation and all calling sites that exist at Z, as well
as new calling sites they add, to be consistent. There is no
bug at A.

Suppose that in the meantime on the lower line of development somebody
adds a new calling site for that function at commit Y. The
commits from Z leading to B all assume the old semantics of that
function and the callers and the callee are consistent with each
other. There is no bug at B, either.

Suppose further that the two development lines merge cleanly at C,
so no conflict resolution is required.

Nevertheless, the code at C is broken, because the callers added
on the lower line of development have not been converted to the new
semantics introduced on the upper line of development. So if all
you know is that D is bad, that Z is good, and that
git-bisect(1) identifies C as the culprit, how will you
figure out that the problem is due to this change in semantics?

When the result of a git bisect is a non-merge commit, you should
normally be able to discover the problem by examining just that commit.
Developers can make this easy by breaking their changes into small
self-contained commits. That won’t help in the case above, however,
because the problem isn’t obvious from examination of any single
commit; instead, a global view of the development is required. To
make matters worse, the change in semantics in the problematic
function may be just one small part of the changes in the upper
line of development.

On the other hand, if instead of merging at C you had rebased the
history between Z to B on top of A, you would have gotten this
linear history:

---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*

Bisecting between Z and D* would hit a single culprit commit Y*,
and understanding why Y* was broken would probably be easier.

Partly for this reason, many experienced git users, even when
working on an otherwise merge-heavy project, keep the history
linear by rebasing against the latest upstream version before
publishing.

Chapter 6. Advanced branch management

Fetching individual branches

Instead of using git-remote(1), you can also choose just
to update one branch at a time, and to store it locally under an
arbitrary name:

$ git fetch origin todo:my-todo-work

The first argument, "origin", just tells git to fetch from the
repository you originally cloned from. The second argument tells git
to fetch the branch named "todo" from the remote repository, and to
store it locally under the name refs/heads/my-todo-work.

You can also fetch branches from other repositories; so

$ git fetch git://example.com/proj.git master:example-master

will create a new branch named "example-master" and store in it the
branch named "master" from the repository at the given URL. If you
already have a branch named example-master, it will attempt to
fast-forward to the commit given by example.com’s
master branch. In more detail:

git fetch and fast-forwards

In the previous example, when updating an existing branch, "git fetch"
checks to make sure that the most recent commit on the remote
branch is a descendant of the most recent commit on your copy of the
branch before updating your copy of the branch to point at the new
commit. Git calls this process a fast-forward.

A fast-forward looks something like this:

o--o--o--o <-- old head of the branch
\
o--o--o <-- new head of the branch

In some cases it is possible that the new head will not actually be
a descendant of the old head. For example, the developer may have
realized she made a serious mistake, and decided to backtrack,
resulting in a situation like:

o--o--o--o--a--b <-- old head of the branch
\
o--o--o <-- new head of the branch

In this case, "git fetch" will fail, and print out a warning.

In that case, you can still force git to update to the new head, as
described in the following section. However, note that in the
situation above this may mean losing the commits labeled "a" and "b",
unless you’ve already created a reference of your own pointing to
them.

Forcing git fetch to do non-fast-forward updates

If git fetch fails because the new head of a branch is not a
descendant of the old head, you may force the update with:

Note the addition of the "+" sign. Alternatively, you can use the "-f"
flag to force updates of all the fetched branches, as in:

$ git fetch -f origin

Be aware that commits that the old version of example/master pointed at
may be lost, as we saw in the previous section.

Configuring remote-tracking branches

We saw above that "origin" is just a shortcut to refer to the
repository that you originally cloned from. This information is
stored in git configuration variables, which you can see using
git-config(1):

The Object Database

We already saw in the section called “Understanding History: Commits” that all commits are stored
under a 40-digit "object name". In fact, all the information needed to
represent the history of a project is stored in objects with such names.
In each case the name is calculated by taking the SHA-1 hash of the
contents of the object. The SHA-1 hash is a cryptographic hash function.
What that means to us is that it is impossible to find two different
objects with the same name. This has a number of advantages; among
others:

Git can quickly determine whether two objects are identical or not,
just by comparing names.

Since object names are computed the same way in every repository, the
same content stored in two repositories will always be stored under
the same name.

Git can detect errors when it reads an object, by checking that the
object’s name is still the SHA-1 hash of its contents.

A "tree" object ties one or more
"blob" objects into a directory structure. In addition, a tree object
can refer to other tree objects, thus creating a directory hierarchy.

A "commit" object ties such directory hierarchies
together into a directed acyclic graph of revisions—each
commit contains the object name of exactly one tree designating the
directory hierarchy at the time of the commit. In addition, a commit
refers to "parent" commit objects that describe the history of how we
arrived at that directory hierarchy.

A "tag" object symbolically identifies and can be
used to sign other objects. It contains the object name and type of
another object, a symbolic name (of course!) and, optionally, a
signature.

The object types in some more detail:

Commit Object

The "commit" object links a physical state of a tree with a description
of how we got there and why. Use the --pretty=raw option to
git-show(1) or git-log(1) to examine your favorite
commit:

a tree: The SHA-1 name of a tree object (as defined below), representing
the contents of a directory at a certain point in time.

parent(s): The SHA-1 name of some number of commits which represent the
immediately previous step(s) in the history of the project. The
example above has one parent; merge commits may have more than
one. A commit with no parents is called a "root" commit, and
represents the initial revision of a project. Each project must have
at least one root. A project can also have multiple roots, though
that isn’t common (or necessarily a good idea).

an author: The name of the person responsible for this change, together
with its date.

a committer: The name of the person who actually created the commit,
with the date it was done. This may be different from the author, for
example, if the author was someone who wrote a patch and emailed it
to the person who used it to create the commit.

a comment describing this commit.

Note that a commit does not itself contain any information about what
actually changed; all changes are calculated by comparing the contents
of the tree referred to by this commit with the trees associated with
its parents. In particular, git does not attempt to record file renames
explicitly, though it can identify cases where the existence of the same
file data at changing paths suggests a rename. (See, for example, the
-M option to git-diff(1)).

A commit is usually created by git-commit(1), which creates a
commit whose parent is normally the current HEAD, and whose tree is
taken from the content currently stored in the index.

Tree Object

The ever-versatile git-show(1) command can also be used to
examine tree objects, but git-ls-tree(1) will give you more
details:

As you can see, a tree object contains a list of entries, each with a
mode, object type, SHA-1 name, and name, sorted by name. It represents
the contents of a single directory tree.

The object type may be a blob, representing the contents of a file, or
another tree, representing the contents of a subdirectory. Since trees
and blobs, like all other objects, are named by the SHA-1 hash of their
contents, two trees have the same SHA-1 name if and only if their
contents (including, recursively, the contents of all subdirectories)
are identical. This allows git to quickly determine the differences
between two related tree objects, since it can ignore any entries with
identical object names.

(Note: in the presence of submodules, trees may also have commits as
entries. See Chapter 8, Submodules for documentation.)

Note that the files all have mode 644 or 755: git actually only pays
attention to the executable bit.

Blob Object

You can use git-show(1) to examine the contents of a blob; take,
for example, the blob in the entry for "COPYING" from the tree above:

$ git show 6ff87c4664

Note that the only valid version of the GPL as far as this project
is concerned is _this_ particular version of the license (ie v2, not
v2.2 or v3.x or whatever), unless explicitly otherwise stated.
...

A "blob" object is nothing but a binary blob of data. It doesn’t refer
to anything else or have attributes of any kind.

Since the blob is entirely defined by its data, if two files in a
directory tree (or in multiple different versions of the repository)
have the same contents, they will share the same blob object. The object
is totally independent of its location in the directory tree, and
renaming a file does not change the object that file is associated with.

Note that any tree or blob object can be examined using
git-show(1) with the <revision>:<path> syntax. This can
sometimes be useful for browsing the contents of a tree that is not
currently checked out.

Trust

If you receive the SHA-1 name of a blob from one source, and its contents
from another (possibly untrusted) source, you can still trust that those
contents are correct as long as the SHA-1 name agrees. This is because
the SHA-1 is designed so that it is infeasible to find different contents
that produce the same hash.

Similarly, you need only trust the SHA-1 name of a top-level tree object
to trust the contents of the entire directory that it refers to, and if
you receive the SHA-1 name of a commit from a trusted source, then you
can easily verify the entire history of commits reachable through
parents of that commit, and all of those contents of the trees referred
to by those commits.

So to introduce some real trust in the system, the only thing you need
to do is to digitally sign just one special note, which includes the
name of a top-level commit. Your digital signature shows others
that you trust that commit, and the immutability of the history of
commits tells others that they can trust the whole history.

In other words, you can easily validate a whole archive by just
sending out a single email that tells the people the name (SHA-1 hash)
of the top commit, and digitally sign that email using something
like GPG/PGP.

To assist in this, git also provides the tag object…

Tag Object

A tag object contains an object, object type, tag name, the name of the
person ("tagger") who created the tag, and a message, which may contain
a signature, as can be seen using git-cat-file(1):

See the git-tag(1) command to learn how to create and verify tag
objects. (Note that git-tag(1) can also be used to create
"lightweight tags", which are not tag objects at all, but just simple
references whose names begin with "refs/tags/").

How git stores objects efficiently: pack files

Newly created objects are initially created in a file named after the
object’s SHA-1 hash (stored in .git/objects).

Unfortunately this system becomes inefficient once a project has a
lot of objects. Try this on an old project:

$ git count-objects
6930 objects, 47620 kilobytes

The first number is the number of objects which are kept in
individual files. The second is the amount of space taken up by
those "loose" objects.

You can save space and make git faster by moving these loose objects in
to a "pack file", which stores a group of objects in an efficient
compressed format; the details of how pack files are formatted can be
found in technical/pack-format.txt.

to remove any of the "loose" objects that are now contained in the
pack. This will also remove any unreferenced objects (which may be
created when, for example, you use "git reset" to remove a commit).
You can verify that the loose objects are gone by looking at the
.git/objects directory or by running

$ git count-objects
0 objects, 0 kilobytes

Although the object files are gone, any commands that refer to those
objects will work exactly as they did before.

The git-gc(1) command performs packing, pruning, and more for
you, so is normally the only high-level command you need.

Dangling objects

The git-fsck(1) command will sometimes complain about dangling
objects. They are not a problem.

The most common cause of dangling objects is that you’ve rebased a
branch, or you have pulled from somebody else who rebased a branch—see
Chapter 5, Rewriting history and maintaining patch series. In that case, the old head of the original
branch still exists, as does everything it pointed to. The branch
pointer itself just doesn’t, since you replaced it with another one.

There are also other situations that cause dangling objects. For
example, a "dangling blob" may arise because you did a "git add" of a
file, but then, before you actually committed it and made it part of the
bigger picture, you changed something else in that file and committed
that updated thing—the old state that you added originally ends up
not being pointed to by any commit or tree, so it’s now a dangling blob
object.

Similarly, when the "recursive" merge strategy runs, and finds that
there are criss-cross merges and thus more than one merge base (which is
fairly unusual, but it does happen), it will generate one temporary
midway tree (or possibly even more, if you had lots of criss-crossing
merges and more than two merge bases) as a temporary internal merge
base, and again, those are real objects, but the end result will not end
up pointing to them, so they end up "dangling" in your repository.

Generally, dangling objects aren’t anything to worry about. They can
even be very useful: if you screw something up, the dangling objects can
be how you recover your old tree (say, you did a rebase, and realized
that you really didn’t want to—you can look at what dangling objects
you have, and decide to reset your head to some old dangling state).

For commits, you can just use:

$ gitk <dangling-commit-sha-goes-here> --not --all

This asks for all the history reachable from the given commit but not
from any branch, tag, or other reference. If you decide it’s something
you want, you can always create a new reference to it, e.g.,

$ git branch recovered-branch <dangling-commit-sha-goes-here>

For blobs and trees, you can’t do the same, but you can still examine
them. You can just do

$ git show <dangling-blob/tree-sha-goes-here>

to show what the contents of the blob were (or, for a tree, basically
what the "ls" for that directory was), and that may give you some idea
of what the operation was that left that dangling object.

Usually, dangling blobs and trees aren’t very interesting. They’re
almost always the result of either being a half-way mergebase (the blob
will often even have the conflict markers from a merge in it, if you
have had conflicting merges that you fixed up by hand), or simply
because you interrupted a "git fetch" with ^C or something like that,
leaving some of the new objects in the object database, but just
dangling and useless.

Anyway, once you are sure that you’re not interested in any dangling
state, you can just prune all unreachable objects:

$ git prune

and they’ll be gone. But you should only run "git prune" on a quiescent
repository—it’s kind of like doing a filesystem fsck recovery: you
don’t want to do that while the filesystem is mounted.

(The same is true of "git fsck" itself, btw, but since
git fsck never actually changes the repository, it just reports
on what it found, git fsck itself is never dangerous to run.
Running it while somebody is actually changing the repository can cause
confusing and scary messages, but it won’t actually do anything bad. In
contrast, running "git prune" while somebody is actively changing the
repository is a BAD idea).

Recovering from repository corruption

By design, git treats data trusted to it with caution. However, even in
the absence of bugs in git itself, it is still possible that hardware or
operating system errors could corrupt data.

The first defense against such problems is backups. You can back up a
git directory using clone, or just using cp, tar, or any other backup
mechanism.

As a last resort, you can search for the corrupted objects and attempt
to replace them by hand. Back up your repository before attempting this
in case you corrupt things even more in the process.

We’ll assume that the problem is a single missing or corrupted blob,
which is sometimes a solvable problem. (Recovering missing trees and
especially commits is much harder).

Before starting, verify that there is corruption, and figure out where
it is with git-fsck(1); this may be time-consuming.

(Typically there will be some "dangling object" messages too, but they
aren’t interesting.)

Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
points to it. If you could find just one copy of that missing blob
object, possibly in some other repository, you could move it into
.git/objects/4b/9458b3… and be done. Suppose you can’t. You can
still examine the tree that pointed to it with git-ls-tree(1),
which might output something like:

So now you know that the missing blob was the data for a file named
"myfile". And chances are you can also identify the directory—let’s
say it’s in "somedirectory". If you’re lucky the missing copy might be
the same as the copy you have checked out in your working tree at
"somedirectory/myfile"; you can test whether that’s right with
git-hash-object(1):

$ git hash-object -w somedirectory/myfile

which will create and store a blob object with the contents of
somedirectory/myfile, and output the SHA-1 of that object. if you’re
extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
which case you’ve guessed right, and the corruption is fixed!

Otherwise, you need more information. How do you tell which version of
the file has been lost?

This tells you that the immediately preceding version of the file was
"newsha", and that the immediately following version was "oldsha".
You also know the commit messages that went with the change from oldsha
to 4b9458b and with the change from 4b9458b to newsha.

If you’ve been committing small enough changes, you may now have a good
shot at reconstructing the contents of the in-between state 4b9458b.

If you can do that, you can now recreate the missing object with

$ git hash-object -w <recreated-file>

and your repository is good again!

(Btw, you could have ignored the fsck, and started with doing a

$ git log --raw --all

and just looked for the sha of the missing object (4b9458b..) in that
whole thing. It’s up to you - git does have a lot of information, it is
just missing one particular blob version.

The index

The index is a binary file (generally kept in .git/index) containing a
sorted list of path names, each with permissions and the SHA-1 of a blob
object; git-ls-files(1) can show you the contents of the index:

Note that in older documentation you may see the index called the
"current directory cache" or just the "cache". It has three important
properties:

The index contains all the information necessary to generate a single
(uniquely determined) tree object.

For example, running git-commit(1) generates this tree object
from the index, stores it in the object database, and uses it as the
tree object associated with the new commit.

The index enables fast comparisons between the tree object it defines
and the working tree.

It does this by storing some additional data for each entry (such as
the last modified time). This data is not displayed above, and is not
stored in the created tree object, but it can be used to determine
quickly which files in the working directory differ from what was
stored in the index, and thus save git from having to read all of the
data from such files to look for changes.

It can efficiently represent information about merge conflicts
between different tree objects, allowing each pathname to be
associated with sufficient information about the trees involved that
you can create a three-way merge between them.

Chapter 8. Submodules

Large projects are often composed of smaller, self-contained modules. For
example, an embedded Linux distribution’s source tree would include every
piece of software in the distribution with some local modifications; a movie
player might need to build against a specific, known-working version of a
decompression library; several independent programs might all share the same
build scripts.

With centralized revision control systems this is often accomplished by
including every module in one single repository. Developers can check out
all modules or only the modules they need to work with. They can even modify
files across several modules in a single commit while moving things around
or updating APIs and translations.

Git does not allow partial checkouts, so duplicating this approach in Git
would force developers to keep a local copy of modules they are not
interested in touching. Commits in an enormous checkout would be slower
than you’d expect as Git would have to scan every directory for changes.
If modules have a lot of local history, clones would take forever.

On the plus side, distributed revision control systems can much better
integrate with external sources. In a centralized model, a single arbitrary
snapshot of the external project is exported from its own revision control
and then imported into the local revision control on a vendor branch. All
the history is hidden. With distributed revision control you can clone the
entire external history and much more easily follow development and re-merge
local changes.

Git’s submodule support allows a repository to contain, as a subdirectory, a
checkout of an external project. Submodules maintain their own identity;
the submodule support just stores the submodule repository location and
commit ID, so other developers who clone the containing project
("superproject") can easily clone all the submodules at the same revision.
Partial checkouts of the superproject are possible: you can tell Git to
clone none, some or all of the submodules.

The git-submodule(1) command is available since Git 1.5.3. Users
with Git 1.5.2 can look up the submodule commits in the repository and
manually check them out; earlier versions won’t recognize the submodules at
all.

To see how submodule support works, create (for example) four example
repositories that can be used later as a submodule:

Note

The commit object names shown above would be different for you, but they
should match the HEAD commit object names of your repositories. You can check
it by running git ls-remote ../a.

Pulling down the submodules is a two-step process. First run git submodule
init to add the submodule repository URLs to .git/config:

$ git submodule init

Now use git submodule update to clone the repositories and check out the
commits specified in the superproject:

$ git submodule update
$ cd a
$ ls -a
. .. .git a.txt

One major difference between git submodule update and git submodule add is
that git submodule update checks out a specific commit, rather than the tip
of a branch. It’s like checking out a tag: the head is detached, so you’re not
working on a branch.

$ git branch
* (no branch)
master

If you want to make a change within a submodule and you have a detached head,
then you should create or checkout a branch, make your changes, publish the
change within the submodule, and then update the superproject to reference the
new commit:

In older git versions it could be easily forgotten to commit new or modified
files in a submodule, which silently leads to similar problems as not pushing
the submodule changes. Starting with git 1.7.0 both "git status" and "git diff"
in the superproject show submodules as modified when they contain new or
modified files to protect against accidentally committing such a state. "git
diff" will also add a "-dirty" to the work tree side when generating patch
output or used with the --submodule option:

Many of the higher-level commands were originally implemented as shell
scripts using a smaller core of low-level git commands. These can still
be useful when doing unusual things with git, or just as a way to
understand its inner workings.

The Workflow

High-level operations such as git-commit(1),
git-checkout(1) and git-reset(1) work by moving data
between the working tree, the index, and the object database. Git
provides low-level operations which perform each of these steps
individually.

Generally, all "git" operations work on the index file. Some operations
work purely on the index file (showing the current state of the
index), but most operations move data between the index file and either
the database or the working directory. Thus there are four main
combinations:

working directory → index

The git-update-index(1) command updates the index with
information from the working directory. You generally update the
index information by just specifying the filename you want to update,
like so:

$ git update-index filename

but to avoid common mistakes with filename globbing etc, the command
will not normally add totally new entries or remove old entries,
i.e. it will normally just update existing cache entries.

To tell git that yes, you really do realize that certain files no
longer exist, or that new files should be added, you
should use the --remove and --add flags respectively.

NOTE! A --remove flag does not mean that subsequent filenames will
necessarily be removed: if the files still exist in your directory
structure, the index will be updated with their new status, not
removed. The only thing --remove means is that update-index will be
considering a removed file to be a valid thing, and if the file really
does not exist any more, it will update the index accordingly.

As a special case, you can also do git update-index --refresh, which
will refresh the "stat" information of each index to match the current
stat information. It will not update the object status itself, and
it will only update the fields that are used to quickly test whether
an object still matches its old backing store object.

index → object database

You write your current index file to a "tree" object with the program

$ git write-tree

that doesn’t come with any options—it will just write out the
current index into the set of tree objects that describe that state,
and it will return the name of the resulting top-level tree. You can
use that tree to re-generate the index at any time by going in the
other direction:

object database → index

You read a "tree" file from the object database, and use that to
populate (and overwrite—don’t do this if your index contains any
unsaved state that you might want to restore later!) your current
index. Normal operation is just

$ git read-tree <SHA-1 of tree>

and your index file will now be equivalent to the tree that you saved
earlier. However, that is only your index file: your working
directory contents have not been modified.

index → working directory

You update your working directory from the index by "checking out"
files. This is not a very common operation, since normally you’d just
keep your files updated, and rather than write to your working
directory, you’d tell the index files about the changes in your
working directory (i.e. git update-index).

However, if you decide to jump to a new version, or check out somebody
else’s version, or just restore a previous tree, you’d populate your
index file with read-tree, and then you need to check out the result
with

$ git checkout-index filename

or, if you want to check out all of the index, use -a.

NOTE! git checkout-index normally refuses to overwrite old files, so
if you have an old version of the tree already checked out, you will
need to use the "-f" flag (before the "-a" flag or the filename) to
force the checkout.

Finally, there are a few odds and ends which are not purely moving
from one representation to the other:

Tying it all together

To commit a tree you have instantiated with "git write-tree", you’d
create a "commit" object that refers to that tree and the history
behind it—most notably the "parent" commits that preceded it in
history.

Normally a "commit" has one parent: the previous state of the tree
before a certain change was made. However, sometimes it can have two
or more parent commits, in which case we call it a "merge", due to the
fact that such a commit brings together ("merges") two or more
previous states represented by other commits.

In other words, while a "tree" represents a particular directory state
of a working directory, a "commit" represents that state in "time",
and explains how we got there.

You create a commit object by giving it the tree that describes the
state at the time of the commit, and a list of parents:

$ git commit-tree <tree> -p <parent> [(-p <parent2>)...]

and then giving the reason for the commit on stdin (either through
redirection from a pipe or file, or by just typing it at the tty).

git commit-tree will return the name of the object that represents
that commit, and you should save it away for later use. Normally,
you’d commit a new HEAD state, and while git doesn’t care where you
save the note about that state, in practice we tend to just write the
result to the file pointed at by .git/HEAD, so that we can always see
what the last committed state was.

Here is an ASCII art by Jon Loeliger that illustrates how
various pieces fit together.

Examining the data

You can examine the data represented in the object database and the
index with various helper tools. For every object, you can use
git-cat-file(1) to examine details about the
object:

$ git cat-file -t <objectname>

shows the type of the object, and once you have the type (which is
usually implicit in where you find the object), you can use

$ git cat-file blob|tree|commit|tag <objectname>

to show its contents. NOTE! Trees have binary content, and as a result
there is a special helper for showing that content, called
git ls-tree, which turns the binary content into a more easily
readable form.

It’s especially instructive to look at "commit" objects, since those
tend to be small and fairly self-explanatory. In particular, if you
follow the convention of having the top commit name in .git/HEAD,
you can do

$ git cat-file commit HEAD

to see what the top commit was.

Merging multiple trees

Git helps you do a three-way merge, which you can expand to n-way by
repeating the merge procedure arbitrary times until you finally
"commit" the state. The normal situation is that you’d only do one
three-way merge (two parents), and commit it, but if you like to, you
can do multiple parents in one go.

To do a three-way merge, you need the two sets of "commit" objects
that you want to merge, use those to find the closest common parent (a
third "commit" object), and then use those commit objects to find the
state of the directory ("tree" object) at these points.

To get the "base" for the merge, you first look up the common parent
of two commits with

$ git merge-base <commit1> <commit2>

which will return you the commit they are both based on. You should
now look up the "tree" objects of those commits, which you can easily
do with (for example)

$ git cat-file commit <commitname> | head -1

since the tree object information is always the first line in a commit
object.

Once you know the three trees you are going to merge (the one "original"
tree, aka the common tree, and the two "result" trees, aka the branches
you want to merge), you do a "merge" read into the index. This will
complain if it has to throw away your old index contents, so you should
make sure that you’ve committed those—in fact you would normally
always do a merge against your last commit (which should thus match what
you have in your current index anyway).

To do the merge, do

$ git read-tree -m -u <origtree> <yourtree> <targettree>

which will do all trivial merge operations for you directly in the
index file, and you can just write the result out with
git write-tree.

Merging multiple trees, continued

Sadly, many merges aren’t trivial. If there are files that have
been added, moved or removed, or if both branches have modified the
same file, you will be left with an index tree that contains "merge
entries" in it. Such an index tree can NOT be written out to a tree
object, and you will have to resolve any such merge clashes using
other tools before you can write out the result.

You can examine such index state with git ls-files --unmerged
command. An example:

Each line of the git ls-files --unmerged output begins with
the blob mode bits, blob SHA-1, stage number, and the
filename. The stage number is git’s way to say which tree it
came from: stage 1 corresponds to $orig tree, stage 2 HEAD
tree, and stage3 $target tree.

Earlier we said that trivial merges are done inside
git read-tree -m. For example, if the file did not change
from $orig to HEAD nor $target, or if the file changed
from $orig to HEAD and $orig to $target the same way,
obviously the final outcome is what is in HEAD. What the
above example shows is that file hello.c was changed from
$orig to HEAD and $orig to $target in a different way.
You could resolve this by running your favorite 3-way merge
program, e.g. diff3, merge, or git’s own merge-file, on
the blob objects from these three stages yourself, like this:

This would leave the merge result in hello.c~2 file, along
with conflict markers if there are conflicts. After verifying
the merge result makes sense, you can tell git what the final
merge result for this file is by:

$ mv -f hello.c~2 hello.c
$ git update-index hello.c

When a path is in the "unmerged" state, running git update-index for
that path tells git to mark the path resolved.

The above is the description of a git merge at the lowest level,
to help you understand what conceptually happens under the hood.
In practice, nobody, not even git itself, runs git cat-file three times
for this. There is a git merge-index program that extracts the
stages to temporary files and calls a "merge" script on it:

$ git merge-index git-merge-one-file hello.c

and that is what higher level git merge -s resolve is implemented with.

Chapter 10. Hacking git

This chapter covers internal details of the git implementation which
probably only git developers need to understand.

Object storage format

All objects have a statically determined "type" which identifies the
format of the object (i.e. how it is used, and how it can refer to other
objects). There are currently four different object types: "blob",
"tree", "commit", and "tag".

Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
that not only specifies their type, but also provides size information
about the data in the object. It’s worth noting that the SHA-1 hash
that is used to name the object is the hash of the original data
plus this header, so sha1sumfile does not match the object name
for file.
(Historical note: in the dawn of the age of git the hash
was the SHA-1 of the compressed object.)

As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
be validated by verifying that (a) their hashes match the content of the
file and (b) the object successfully inflates to a stream of bytes that
forms a sequence of <ascii type without space> + <space> + <ascii decimal
size> + <byte\0> + <binary object data>.

The structured objects can further have their structure and
connectivity to other objects verified. This is generally done with
the git fsck program, which generates a full dependency graph
of all objects, and verifies their internal consistency (in addition
to just verifying their superficial consistency through the hash).

A birds-eye view of Git’s source code

It is not always easy for new developers to find their way through Git’s
source code. This section gives you a little guidance to show where to
start.

A good place to start is with the contents of the initial commit, with:

$ git checkout e83c5163

The initial revision lays the foundation for almost everything git has
today, but is small enough to read in one sitting.

Note that terminology has changed since that revision. For example, the
README in that revision uses the word "changeset" to describe what we
now call a commit.

Also, we do not call it "cache" any more, but rather "index"; however, the
file is still called cache.h. Remark: Not much reason to change it now,
especially since there is no good single name for it anyway, because it is
basically the header file which is included by all of Git’s C sources.

If you grasp the ideas in that initial commit, you should check out a
more recent version and skim cache.h, object.h and commit.h.

In the early days, Git (in the tradition of UNIX) was a bunch of programs
which were extremely simple, and which you used in scripts, piping the
output of one into another. This turned out to be good for initial
development, since it was easier to test new things. However, recently
many of these parts have become builtins, and some of the core has been
"libified", i.e. put into libgit.a for performance, portability reasons,
and to avoid code duplication.

By now, you know what the index is (and find the corresponding data
structures in cache.h), and that there are just a couple of object types
(blobs, trees, commits and tags) which inherit their common structure from
struct object, which is their first member (and thus, you can cast e.g.
(struct object *)commit to achieve the same as &commit→object, i.e.
get at the object name and flags).

Now is a good point to take a break to let this information sink in.

Next step: get familiar with the object naming. Read the section called “Naming commits”.
There are quite a few ways to name an object (and not only revisions!).
All of these are handled in sha1_name.c. Just have a quick look at
the function get_sha1(). A lot of the special handling is done by
functions like get_sha1_basic() or the likes.

This is just to get you into the groove for the most libified part of Git:
the revision walker.

git rev-list is the original version of the revision walker, which
always printed a list of revisions to stdout. It is still functional,
and needs to, since most new Git commands start out as scripts using
git rev-list.

git rev-parse is not as important any more; it was only used to filter out
options that were relevant for the different plumbing commands that were
called by the script.

Most of what git rev-list did is contained in revision.c and
revision.h. It wraps the options in a struct named rev_info, which
controls how and what revisions are walked, and more.

The original job of git rev-parse is now taken by the function
setup_revisions(), which parses the revisions and the common command line
options for the revision walker. This information is stored in the struct
rev_info for later consumption. You can do your own command line option
parsing after calling setup_revisions(). After that, you have to call
prepare_revision_walk() for initialization, and then you can get the
commits one by one with the function get_revision().

If you are interested in more details of the revision walking process,
just have a look at the first implementation of cmd_log(); call
git show v1.3.0~155^2~4 and scroll down to that function (note that you
no longer need to call setup_pager() directly).

Nowadays, git log is a builtin, which means that it is contained in the
command git. The source side of a builtin is

a function called cmd_<bla>, typically defined in builtin-<bla>.c,
and declared in builtin.h,

an entry in the commands[] array in git.c, and

an entry in BUILTIN_OBJECTS in the Makefile.

Sometimes, more than one builtin is contained in one source file. For
example, cmd_whatchanged() and cmd_log() both reside in builtin-log.c,
since they share quite a bit of code. In that case, the commands which are
not named like the .c file in which they live have to be listed in
BUILT_INS in the Makefile.

git log looks more complicated in C than it does in the original script,
but that allows for a much greater flexibility and performance.

Here again it is a good point to take a pause.

Lesson three is: study the code. Really, it is the best way to learn about
the organization of Git (after you know the basic concepts).

So, think about something which you are interested in, say, "how can I
access a blob just knowing the object name of it?". The first step is to
find a Git command with which you can do it. In this example, it is either
git show or git cat-file.

For the sake of clarity, let’s stay with git cat-file, because it

is plumbing, and

was around even in the initial commit (it literally went only through
some 20 revisions as cat-file.c, was renamed to builtin-cat-file.c
when made a builtin, and then saw less than 10 versions).

So, look into builtin-cat-file.c, search for cmd_cat_file() and look what
it does.

Let’s skip over the obvious details; the only really interesting part
here is the call to get_sha1(). It tries to interpret argv[2] as an
object name, and if it refers to an object which is present in the current
repository, it writes the resulting SHA-1 into the variable sha1.

Two things are interesting here:

get_sha1() returns 0 on success. This might surprise some new
Git hackers, but there is a long tradition in UNIX to return different
negative numbers in case of different errors—and 0 on success.

the variable sha1 in the function signature of get_sha1() is unsigned
char *, but is actually expected to be a pointer to unsigned
char[20]. This variable will contain the 160-bit SHA-1 of the given
commit. Note that whenever a SHA-1 is passed as unsigned char *, it
is the binary representation, as opposed to the ASCII representation in
hex characters, which is passed as char *.

You will see both of these things throughout the code.

Now, for the meat:

case 0:
buf = read_object_with_reference(sha1, argv[1], &size, NULL);

This is how you read a blob (actually, not only a blob, but any type of
object). To know how the function read_object_with_reference() actually
works, find the source code for it (something like git grep
read_object_with | grep ":[a-z]" in the git repository), and read
the source.

To find out how the result can be used, just read on in cmd_cat_file():

write_or_die(1, buf, size);

Sometimes, you do not know where to look for a feature. In many such cases,
it helps to search through the output of git log, and then git show the
corresponding commit.

Example: If you know that there was some test case for git bundle, but
do not remember where it was (yes, you couldgit grep bundle t/, but that
does not illustrate the point!):

$ git log --no-merges t/

In the pager (less), just search for "bundle", go a few lines back,
and see that it is in commit 18449ab0… Now just copy this object name,
and paste it into the command line

$ git show 18449ab0

Voila.

Another example: Find out what to do in order to make some script a
builtin:

$ git log --no-merges --diff-filter=A builtin-*.c

You see, Git is actually the best tool to find out about the source of Git
itself!

Chapter 11. Git Glossary

alternate object database

Via the alternates mechanism, a repository
can inherit part of its object database
from another object database, which is called "alternate".

bare repository

A bare repository is normally an appropriately
named directory with a .git suffix that does not
have a locally checked-out copy of any of the files under
revision control. That is, all of the git
administrative and control files that would normally be present in the
hidden .git sub-directory are directly present in the
repository.git directory instead,
and no other files are present and checked out. Usually publishers of
public repositories make bare repositories available.

A "branch" is an active line of development. The most recent
commit on a branch is referred to as the tip of
that branch. The tip of the branch is referenced by a branch
head, which moves forward as additional development
is done on the branch. A single git
repository can track an arbitrary number of
branches, but your working tree is
associated with just one of them (the "current" or "checked out"
branch), and HEAD points to that branch.

In SCM jargon, "cherry pick" means to choose a subset of
changes out of a series of changes (typically commits) and record them
as a new series of changes on top of a different codebase. In GIT, this is
performed by the "git cherry-pick" command to extract the change introduced
by an existing commit and to record it based on the tip
of the current branch as a new commit.

As a noun: A single point in the
git history; the entire history of a project is represented as a
set of interrelated commits. The word "commit" is often
used by git in the same places other revision control systems
use the words "revision" or "version". Also used as a short
hand for commit object.

As a verb: The action of storing a new snapshot of the project’s
state in the git history, by creating a new commit representing the current
state of the index and advancing HEAD
to point at the new commit.

Directed acyclic graph. The commit objects form a
directed acyclic graph, because they have parents (directed), and the
graph of commit objects is acyclic (there is no chain
which begins and ends with the same object).

Normally the HEAD stores the name of a
branch. However, git also allows you to check out
an arbitrary commit that isn’t necessarily the tip of any
particular branch. In this case HEAD is said to be "detached".

Favorite synonym to "tree-ish" by some total geeks. See
http://en.wikipedia.org/wiki/Ent_(Middle-earth) for an in-depth
explanation. Avoid this term, not to confuse people.

evil merge

An evil merge is a merge that introduces changes that
do not appear in any parent.

fast-forward

A fast-forward is a special type of merge where you have a
revision and you are "merging" another
branch's changes that happen to be a descendant of what
you have. In such these cases, you do not make a new mergecommit but instead just update to his
revision. This will happen frequently on a
remote-tracking branch of a remote
repository.

Grafts enables two otherwise different lines of development to be joined
together by recording fake ancestry information for commits. This way
you can make git pretend the set of parents a commit has
is different from what was recorded when the commit was
created. Configured via the .git/info/grafts file.

The current branch. In more detail: Your working tree is normally derived from the state of the tree
referred to by HEAD. HEAD is a reference to one of the
heads in your repository, except when using a
detached HEAD, in which case it directly
references an arbitrary commit.

During the normal execution of several git commands, call-outs are made
to optional scripts that allow a developer to add functionality or
checking. Typically, the hooks allow for a command to be pre-verified
and potentially aborted, and allow for a post-notification after the
operation is done. The hook scripts are found in the
$GIT_DIR/hooks/ directory, and are enabled by simply
removing the .sample suffix from the filename. In earlier versions
of git you had to make them executable.

index

A collection of files with stat information, whose contents are stored
as objects. The index is a stored version of your
working tree. Truth be told, it can also contain a second, and even
a third version of a working tree, which are used
when merging.

index entry

The information regarding a particular file, stored in the
index. An index entry can be unmerged, if a
merge was started, but not yet finished (i.e. if
the index contains multiple versions of that file).

master

The default development branch. Whenever you
create a git repository, a branch named
"master" is created, and becomes the active branch. In most
cases, this contains the local development, though that is
purely by convention and is not required.

merge

As a verb: To bring the contents of another
branch (possibly from an external
repository) into the current branch. In the
case where the merged-in branch is from a different repository,
this is done by first fetching the remote branch
and then merging the result into the current branch. This
combination of fetch and merge operations is called a
pull. Merging is performed by an automatic process
that identifies changes made since the branches diverged, and
then applies all those changes together. In cases where changes
conflict, manual intervention may be required to complete the
merge.

As a noun: unless it is a fast-forward, a
successful merge results in the creation of a new commit
representing the result of the merge, and having as
parents the tips of the merged branches.
This commit is referred to as a "merge commit", or sometimes just a
"merge".

object

The unit of storage in git. It is uniquely identified by the
SHA1 of its contents. Consequently, an
object can not be changed.

object database

Stores a set of "objects", and an individual object is
identified by its object name. The objects usually
live in $GIT_DIR/objects/.

To merge more than two branches. Also denotes an
intelligent predator.

origin

The default upstream repository. Most projects have
at least one upstream project which they track. By default
origin is used for that purpose. New upstream updates
will be fetched into remote remote-tracking branches named
origin/name-of-upstream-branch, which you can see using
git branch -r.

pack

A set of objects which have been compressed into one file (to save space
or to transmit them efficiently).

pack index

The list of identifiers, and other information, of the objects in a
pack, to assist in efficiently accessing the contents of a
pack.

pathspec

Pattern used to specify paths.

Pathspecs are used on the command line of "git ls-files", "git
ls-tree", "git add", "git grep", "git diff", "git checkout",
and many other commands to
limit the scope of operations to some subset of the tree or
worktree. See the documentation of each command for whether
paths are relative to the current directory or toplevel. The
pathspec syntax is as follows:

any path matches itself

the pathspec up to the last slash represents a
directory prefix. The scope of that pathspec is
limited to that subtree.

the rest of the pathspec is a pattern for the remainder
of the pathname. Paths relative to the directory
prefix will be matched against that pattern using fnmatch(3);
in particular, * and ?can match directory separators.

For example, Documentation/*.jpg will match all .jpg files
in the Documentation subtree,
including Documentation/chapter_1/figure_1.jpg.

A pathspec that begins with a colon : has special meaning. In the
short form, the leading colon : is followed by zero or more "magic
signature" letters (which optionally is terminated by another colon :),
and the remainder is the pattern to match against the path. The optional
colon that terminates the "magic signature" can be omitted if the pattern
begins with a character that cannot be a "magic signature" and is not a
colon.

In the long form, the leading colon : is followed by a open
parenthesis (, a comma-separated list of zero or more "magic words",
and a close parentheses ), and the remainder is the pattern to match
against the path.

The "magic signature" consists of an ASCII symbol that is not
alphanumeric.

top /

The magic word top (mnemonic: /) makes the pattern match
from the root of the working tree, even when you are running
the command from inside a subdirectory.

Currently only the slash / is recognized as the "magic signature",
but it is envisioned that we will support more types of magic in later
versions of git.

A pathspec with only a colon means "there is no pathspec". This form
should not be combined with other pathspec.

parent

A commit object contains a (possibly empty) list
of the logical predecessor(s) in the line of development, i.e. its
parents.

pickaxe

The term pickaxe refers to an option to the diffcore
routines that help select changes that add or delete a given text
string. With the --pickaxe-all option, it can be used to view the full
changeset that introduced or removed, say, a
particular line of text. See git-diff(1).

Pushing a branch means to get the branch’s
head ref from a remote repository,
find out if it is a direct ancestor to the branch’s local
head ref, and in that case, putting all
objects, which are reachable from the local
head ref, and which are missing from the remote
repository, into the remote
object database, and updating the remote
head ref. If the remote head is not an
ancestor to the local head, the push fails.

reachable

All of the ancestors of a given commit are said to be
"reachable" from that commit. More
generally, one object is reachable from
another if we can reach the one from the other by a chain
that follows tags to whatever they tag,
commits to their parents or trees, and
trees to the trees or blobs
that they contain.

rebase

To reapply a series of changes from a branch to a
different base, and reset the head of that branch
to the result.

ref

A 40-byte hex representation of a SHA1 or a name that
denotes a particular object. They may be stored in
a file under $GIT_DIR/refs/ directory, or
in the $GIT_DIR/packed-refs file.

reflog

A reflog shows the local "history" of a ref. In other words,
it can tell you what the 3rd last revision in this repository
was, and what was the current state in this repository,
yesterday 9:14pm. See git-reflog(1) for details.

refspec

A "refspec" is used by fetch and
push to describe the mapping between remote
ref and local ref. They are combined with a colon in
the format <src>:<dst>, preceded by an optional plus sign, +.
For example: git fetch $URL
refs/heads/master:refs/heads/origin means "grab the master
branchhead from the $URL and store
it as my origin branch head". And git push
$URL refs/heads/master:refs/heads/to-upstream means "publish my
master branch head as to-upstream branch at $URL". See also
git-push(1).

remote-tracking branch

A regular git branch that is used to follow changes from
another repository. A remote-tracking
branch should not contain direct modifications or have local commits
made to it. A remote-tracking branch can usually be
identified as the right-hand-side ref in a Pull:
refspec.

A shallow repository has an incomplete
history some of whose commits have parents cauterized away (in other
words, git is told to pretend that these commits do not have the
parents, even though they are recorded in the commit object). This is sometimes useful when you are interested only in the
recent history of a project even though the real history recorded in the
upstream is much larger. A shallow repository
is created by giving the --depth option to git-clone(1), and
its history can be later deepened with git-fetch(1).

symref

Symbolic reference: instead of containing the SHA1
id itself, it is of the format ref: refs/some/thing and when
referenced, it recursively dereferences to this reference.
HEAD is a prime example of a symref. Symbolic
references are manipulated with the git-symbolic-ref(1)
command.

tag

A ref under refs/tags/ namespace that points to an
object of an arbitrary type (typically a tag points to either a
tag or a commit object).
In contrast to a head, a tag is not updated by
the commit command. A git tag has nothing to do with a Lisp
tag (which would be called an object type
in git’s context). A tag is most typically used to mark a particular
point in the commit ancestry chain.

tag object

An object containing a ref pointing to
another object, which can contain a message just like a
commit object. It can also contain a (PGP)
signature, in which case it is called a "signed tag object".

topic branch

A regular git branch that is used by a developer to
identify a conceptual line of development. Since branches are very easy
and inexpensive, it is often desirable to have several small branches
that each contain very well defined concepts or small incremental yet
related changes.

tree

Either a working tree, or a tree object together with the dependent blob and tree objects
(i.e. a stored representation of a working tree).

tree object

An object containing a list of file names and modes along
with refs to the associated blob and/or tree objects. A
tree is equivalent to a directory.

The default branch that is merged into the branch in
question (or the branch in question is rebased onto). It is configured
via branch.<name>.remote and branch.<name>.merge. If the upstream branch
of A is origin/B sometimes we say "A is tracking origin/B".

working tree

The tree of actual checked out files. The working tree normally
contains the contents of the HEAD commit’s tree,
plus any local changes that you have made but not yet committed.

$ git bisect start
$ git bisect bad # current version is bad
$ git bisect good v2.6.13-rc2 # last known good revision
Bisecting: 675 revisions left to test after this
# test here, then:
$ git bisect good # if this revision is good, or
$ git bisect bad # if this revision is bad.
# repeat until done.

Repository maintenance

Check for corruption:

$ git fsck

Recompress, remove unused cruft:

$ git gc

Appendix B. Notes and todo list for this manual

This is a work in progress.

The basic requirements:

It must be readable in order, from beginning to end, by someone
intelligent with a basic grasp of the UNIX command line, but without
any special knowledge of git. If necessary, any other prerequisites
should be specifically mentioned as they arise.

Whenever possible, section headings should clearly describe the task
they explain how to do, in language that requires no more knowledge
than necessary: for example, "importing patches into a project" rather
than "the git am command"

Think about how to create a clear chapter dependency graph that will
allow people to get to important topics without necessarily reading
everything in between.